A diagram of a conventional power amplifying apparatus is shown in FIG. 4. As shown in the figure, the power amplifying apparatus is configured by an RC (Raised Cosine) filter 110, DA converters (hereinafter, DACs) 120i, 120q, smoothing filters (hereinafter, SMFs) 130i, 130q, an orthogonal modulating section 140, and a linear power amplifier 150.
The digital RC filter 110 conducts base-band band limitation on each of an in-phase component (hereinafter, I signal) and orthogonal component (hereinafter, Q signal) of an input signal. The DA converters 120i, 120q convert the I and Q signals output from the digital RC filter 110, to digital signals, respectively. The SMFs 130i, 130q remove alias from signals output from the DACs 120i, 120q, respectively.
The orthogonal modulating section 140 performs orthogonal modulation on outputs of the SMFs 130i, 130q, and up-converts the modulated outputs to an RF signal. As the method of the orthogonal modulation, useful are the superheterodyne method which uses an orthogonal modulator and a mixer, the direct modulation method, and the like. The linear power amplifier 150 amplifies a modulated signal from the orthogonal modulating section 140. In this way, an output of the power amplifying apparatus is obtained.
In the power amplifying apparatus shown in FIG. 4, since a linear power amplifier is used as the amplifier 150, there is a circumstance where the efficiency of the output signal with respect to the input signal is low.
Therefore, the LINC (Linear Amplification with Nonlinear Components) method which uses a nonlinear power amplifier to conduct linear amplification attracts attention. In the LINC method, since a nonlinear power amplifier is used, the efficiency of the output signal with respect to the input signal is enhanced.
FIG. 5 shows a power amplifying apparatus using the LINC method. As shown in the figure, the power amplifying apparatus using the LING method is configured by a signal converting circuit 210, a voltage controlled oscillator (hereinafter, VCO) 220, and a nonlinear power amplifier 230.
The signal converting circuit 210 converts the I and Q signals of the input signal from the orthogonal coordinate system to the polar coordinate system, and outputs an amplitude component A(t) and phase component θ(t) of the polar coordinate signal. The conversion from the orthogonal coordinate system to the polar coordinate system is indicated by Exp. (1):I(t)+jQ(t)=A(t)exp(jθ(t))  (1)
Furthermore, the signal converting circuit 210 applies directly modulation on the VCO 220 with using the phase component q(t). The VCO 220 performs phase modulation on the basis of q(t), and outputs a modulated signal. The signal modulated by the VCO 220 is input to the nonlinear power amplifier 230, and also to the signal converting circuit 210 in order to compensate a VCO control signal.
Moreover, the signal converting circuit 210 controls the gain of the nonlinear power amplifier 230 on the basis of the amplitude component A(t).
On the basis of the amplitude component A(t) output from the signal converting circuit 210, the nonlinear power amplifier 230 amplifies the amplitude component A(t) output from the signal converting circuit 210, and the modulated signal output from VCO 220.
According to the configuration shown FIG. 5, the signal which has undergone phase modulation has a very low peak average power ratio (hereinafter, PAR), and hence is not distorted even when a nonlinear power amplifier is used. Therefore, a nonlinear power amplifier can be used, and the efficiency of the output signal with respect to the input signal can be made higher than the case where a linear power amplifier is used. Moreover, the signal converting circuit 210 can be integrated into one chip, and a power amplifying apparatus, and a communication terminal apparatus on which the power amplifying apparatus is mounted can be miniaturized and reduced in cost.
In the case where the power amplifying apparatus of the configuration shown in FIG. 5 is applied to a signal having a high dynamic range, however, the linearity of the gain control cannot be held. Consequently, there is a circumstance where such a power amplifying apparatus cannot be applied to a base station in a third-generation communication system using the CDMA system or the like, or base and mobile stations in a fourth-generation communication system (using the OFDM system or the like) in which a signal having a high dynamic range is required.
In the case where a power amplifying apparatus is applied to a broadbanded system such as a base station in a third-generation communication system or base and mobile stations in a fourth-generation communication system, moreover, the response speed of a VCO cannot follow. Consequently, there is a circumstance where a phase-modulated signal is distorted, and distortion characteristics such as the adjacent channel leakage power ratio (ACLR) characteristic are impaired.